There is only one Mark Thomas in the "Index of Rev. War Pension Applications in the National Archives". You probably already have this information but just in case you don't:
Thomas, Mark, Va., R18446: Va. Half Pay. *See N.A. Acc. No. 874-050172 Half Pay, Mark Thomas.
Note: The "R" infront of the pension application # (in this case) probably means the last heir/widow who applied was rejected possibly due to lack of documentation to prove relationship to soldier.
*"These instructions refer to a separate series of pension application files that are also in Record Group 15. The records in these files relate mainly to claims made by heirs under an act of Congress approved July 5, 1832 (4 Stat. 563), which provided half-pay pensions for Virginia veterans of the Revolutionary War, and to claims made by heirs for arrears de veterans of Virginia and other States under other pension acts...Abstracts of their contents are in Gaius Marcus Brumbaugh, "Revolutionary Ward Records, Volume 1: Virginia (Washington, 1936)."
Hope you find something here that you didn't already have.